Quark spins and anomalous Ward identity

We calculate the intrinsic quark spin contribution to the total proton spin using overlap valence quarks on three ensembles of 2 +1 -flavor RBC/UKQCD domain-wall configurations with different lattice spacings. The lowest pion mass of the ensembles is around 171 MeV, which is close to the physical point. With overlap fermions and a topological charge derived from the overlap operator, we verify the anomalous Ward identity between nucleon states with momentum transfer. Both the connected and the disconnected insertions of the axial-vector current are calculated. For the disconnected-insertion part, the cluster-decomposition error reduction technique is utilized for the lattice with the largest volume and the error can be reduced by 10%-40%. Nonperturbative renormalization is carried out and the final results are all reported in the MS ¯ scheme at 2 GeV. We determine the total quark spin contribution to the nucleon spin to be Δ Σ =0.405 (25 )(37 ) , which is consistent with the recent global fitting result of experimental data. The isovector axial coupling we obtain in this study is g_A³=1.254 (16 )(30 ) , which agrees well with the experimental value of 1.2723(23).